Method for forming double patterned structure

ABSTRACT

A semiconductor structure including a double patterned structure and a method for forming the semiconductor structure are provided. A positive photoresist layer is formed on a negative photoresist layer, which is formed over a substrate. An exposure process is performed to form a first exposure region in the positive photoresist layer and to form a second exposure region in the negative photoresist layer in response to a first and a second intensity thresholds of the exposure energy. A positive-tone development process is performed to remove the first exposure region from the positive photoresist layer to form first opening(s). The second exposure region in the negative photoresist layer is then etched along the first opening(s) to form second opening(s) therein. A negative-tone development process is performed to remove portions of the negative photoresist layer outside of remaining second exposure region to form a double patterned negative photoresist layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Chinese patent application No.CN201210062523.X, filed on Mar. 9, 2012 and entitled “METHOD FOR FORMINGSEMICONDUCTOR STRUCTURE”, the entire disclosure of which is incorporatedherein by reference.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to the field of semiconductormanufacturing and, more particularly, to a double patterned structureand a method for forming the double patterned structure.

BACKGROUND OF THE DISCLOSURE

In semiconductor device manufacturing, lithographic and etchingprocesses are repeatedly performed to form patterned structures. Forexample, a photoresist layer may be firstly formed on a substrate to beprocessed, then exposure and development processes are performed to formopenings in the photoresist layer. The substrate is exposed through theopenings. Thereafter, the substrate is etched by using the patternedphotoresist layer as a mask to form a desired pattern in the substrate.

In the development process, portions of the photoresist are exposed bylight penetrating through a photo mask and the chemical characteristicsof the exposed portions of the photoresist layer may be changed. In acase that a positive photoresist layer is used, the exposed portions arealtered from insoluble to soluble. In a case that a negative photoresistlayer is used, the exposed portions are altered from soluble toinsoluble. Therefore, some portions of the photoresist layer are removedin the development process and the pattern on the photo mask istransferred onto the photoresist layer.

In a practical manufacturing process, the smallest distance between twoneighboring components of a final device (the distance is named as“pitch”) depends on a resolution ratio of the exposure system. Thesmaller the ratio is, the smaller the pitch can be. With smaller pitch,semiconductor devices can be better integrated.

Conventional methods for reducing a pitch in a semiconductor deviceinclude use of a double patterning technology, which includeslitho-etch-litho-etch (LELE) processes and dual-tone development (DDT)processes.

In a LELE process, two lithographic processes and two etching processesare performed to one substrate to form a pattern thereon. Although, byusing the LELE process, the pitch between two neighboring components inthe pattern may be smaller, processing complexity is added because twolithographic processes and two etching processes are performed.

In a DDT process, a photoresist layer is formed on a substrate and thenexposed to form a dual pattern. Then, a positive-tone development and anegative-tone development are performed to the photoresist layer havingthe dual pattern therein. Such process is difficult to control becausethe positive-tone development and the negative-tone development mayaffect each other.

Therefore, there is a need to provide a double patterned structure and asimplified method for forming the double patterned structure with easycontrol of formation.

SUMMARY

According to various embodiments, there is provided a method for forminga semiconductor structure. The semiconductor structure can be formed byforming a negative photoresist layer over a substrate and forming apositive photoresist layer on the negative photoresist layer. Anexposure process can be performed to form a first exposure region in thepositive photoresist layer and to form a second exposure region in thenegative photoresist layer. A positive-tone development process can beformed to remove the first exposure region from the positive photoresistlayer to form one or more first openings to expose the negativephotoresist layer. The second exposure region of the negativephotoresist layer can be etched along the one or more first openings toform one or more second openings through both the positive photoresistlayer and the negative photoresist layer. Remaining positive photoresistlayer can be removed. A negative-tone development process can performedto remove portions of the negative photoresist layer outside ofremaining second exposure region to form a double patterned negativephotoresist layer.

According to various embodiments, there is also provided a semiconductorstructure. The semiconductor structure can include a substrate and adouble patterned negative photoresist layer disposed over the substrate.The double patterned negative photoresist layer can be formed by forminga negative photoresist layer over a substrate and forming a positivephotoresist layer on the negative photoresist layer. An exposure processcan be performed to form a first exposure region in the positivephotoresist layer and to form a second exposure region in the negativephotoresist layer. A positive-tone development process can be formed toremove the first exposure region from the positive photoresist layer toform one or more first openings to expose the negative photoresistlayer. The second exposure region of the negative photoresist layer canbe etched along the one or more first openings to form one or moresecond openings through both the positive photoresist layer and thenegative photoresist layer. Remaining positive photoresist layer can beremoved. A negative-tone development process can performed to removeportions of the negative photoresist layer outside of remaining secondexposure region to form a double patterned negative photoresist layer.

In some embodiments, removing the remaining positive photoresist layer,e.g., in a positive-tone development process or any suitable processes,and etching the second exposure region of the negative photoresist layeralong the first openings are performed simultaneously. Alternatively,the remaining positive photoresist layer may be removed in thenegative-tone development process to further reduce processing steps andsave manufacturing cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-7 are cross-sectional views of intermediate structuresillustrating a current LELE process for forming a double patternedstructure;

FIG. 8 is a flow chart of an exemplary method for forming a doublepatterned structure according to various disclosed embodiments; and

FIGS. 9-16 are cross-sectional views of intermediate structuresillustrating a process for forming a double patterned structureaccording to various disclosed embodiments.

DETAILED DESCRIPTION OF THE DISCLOSURE

Reference will now be made in detail to exemplary embodiments of thedisclosure, which are illustrated in the accompanying drawings. Whereverpossible, the same reference numbers will be used throughout thedrawings to refer to the same or like parts. For illustration purposes,elements illustrated in the accompanying drawings are not drawn toscale, which are not intended to limit the scope of the presentdisclosure. In practical operations, each element in the drawings hasspecific dimensions such as a length, a width, and a depth.

FIGS. 1-7 are cross-sectional views of intermediate structureillustrating a current litho-etch-litho-etch (LELE) process for forminga double patterned structure.

Referring to FIG. 1, a substrate 100 is provided. A target layer 109 isformed on the substrate 100. A first photoresist layer 101 is formed onthe target layer 109. A first photo mask 20 with a nonopaque part 21 andan opaque part 22 is provided. Exposure light 30 penetrates through thenonopaque part 21 to expose the first photoresist layer 101 to form afirst exposure region 102 therein.

Referring to FIG. 2, a first photoresist layer 101 a having one or morefirst openings 103 is formed, after a first development process isperformed to remove the first exposure region 102 (shown in FIG. 1).

Referring to FIG. 3, the target layer 109 is etched along the firstopenings 103 using the first photoresist layer 101 a as a mask to formone or more second openings 104.

Referring to FIG. 4, the first photoresist layer 101 a (shown in FIG. 3)is removed. A second photoresist layer 105 is formed on the target layer109 and the substrate 100. A second photo mask 40 with a nonopaque part41 and an opaque part 42 is provided. The exposure light 30 penetratesthrough the nonopaque part 41 to expose the second photoresist layer 105to form a second exposure region 106 therein.

Referring to FIG. 5, a second photoresist layer 105 a having one or morethird openings 107 is formed after a second development process isperformed to remove the second exposure region 106 (shown in FIG. 4).

Referring to FIG. 6, the target layer 109 is etched along the thirdopenings 107 using the second photoresist layer 105 a as a mask to formone or more fourth openings 108 in the target layer 109.

Referring to FIG. 7, the second photoresist layer 105 a (shown in FIG.6) is removed. As such, by employing the LELE process, a doublepatterned target layer is obtained by performing two lithographicprocesses and two etching processes. However, the process is complex andexpensive.

A simplified method for forming a double patterned structure isprovided. In the method, a substrate is provided and a negativephotoresist layer is formed over the substrate. A positive photoresistlayer is formed over the negative photoresist layer. An exposure processis performed to expose the negative photoresist layer and the positivephotoresist layer with an exposure energy having a first intensitythreshold and a second intensity threshold. The first intensitythreshold is higher than the second intensity threshold. A firstexposure region is formed in the positive photoresist layer in responseto the exposure energy having the first intensity threshold, and asecond exposure region is formed in the negative photoresist layer inresponse to the exposure energy having the second intensity threshold.The second exposure region is wider than the first exposure region.Thereafter, a first development process (e.g., a positive-tonedevelopment process) is performed to remove the first exposure regionfrom the positive photoresist layer to form first opening(s) to exposeportions of the second exposure region of the negative photoresistlayer. The second exposure region of the negative photoresist layer isetched along the first opening(s) to form second opening(s) through boththe positive photoresist layer and the negative photoresist layer and toexpose an underlying layer, such as the substrate. The remainingpositive photoresist layer in the first development process is removed.And then, a second development process (e.g., a negative-tonedevelopment process) is performed to remove portions of the negativephotoresist layer outside of remaining second exposure region.Therefore, a double patterned negative photoresist layer is formed.

In this manner, the disclosed method uses one exposure process and onephoto mask to save the processing cost. In the one exposure process, afirst exposure region and a second exposure region are respectivelyformed in the positive photoresist layer and the negative photoresistlayer. A pattern of the positive photoresist layer is transferred to thenegative photoresist layer by an etching process, and the pattern of thenegative photoresist layer is formed by a negative-tone developmentprocess. Therefore, a double patterned structure is formed in thenegative photoresist layer. The process is simple with high accuracy. Inaddition, because development processes are respectively performed tothe negative photoresist layer and the positive photoresist layer, thereare no interactions between the development processes. Such processesare easily controlled with good topography for obtaining the doublepatterned structure.

FIG. 8 is a flow chart of an exemplary method for forming a doublepatterned structure according to various disclosed embodiments. Theexemplary method depicted in FIG. 8 is illustrated herein in detail withreference to the accompanying drawings, including, e.g., FIGS. 9-16.Specifically, FIGS. 9-16 are cross-sectional views of intermediatestructures illustrating a process for forming a double patternedstructure according to various disclosed embodiments.

In Step S201 of FIG. 8 and referring to FIG. 9, a substrate 300 isprovided.

The substrate 300 may include monocrystalline silicon, monocrystallinegermanium, GeSi, SiC, and/or III-V group compounds such as GaAs, InP, orthe like. For example, the substrate 300 may be a silicon-on-insulator(SOI) substrate or a germanium-on-insulator (GOI) substrate.Specifically, semiconductor devices may be formed on the substrate 300including, for example, MOS transistors, diodes, capacitances,inductances, and the like.

In Step S202 of FIG. 8 and still referring to FIG. 9, a target layer310, an anti-reflection coating (ARC) layer 301, a negative photoresistlayer 302, and a positive photoresist layer 303 are successively formedon the substrate 300 as shown in FIG. 9.

The target layer 310 is formed on a top surface of the substrate 300.The target layer 310 may include an insulating material including, forexample, silicon dioxide, silicon nitride, silicon carbide, siliconoxynitride, and/or the like. Alternatively, the target layer 310 mayinclude a conductive or semi-conductive material including, for example,metal, metal oxide, metal nitride, metal oxynitride, metal silicide,silicon, polysilicon, and/or the like. Of course, the target layer 310may include any other suitable materials.

The target layer 310 may include a single-layer structure or amulti-layer stack structure. The target layer 310 may include at leastone of the conductive material and/or the insulating material.

The ARC layer 301 is formed on the target layer 310 by applying aspin-on process or a deposition process. The ARC layer 301 may have athickness in a range from about 100 Å to about 1500 Å. The ARC layer 301is adapted for eliminating standing wave effect, which may occur in asubsequent exposure process as a result of the optical reflection andoptical interference. This allows subsequent-formed photoresist layersto have desired topography, e.g., for sidewalls.

The ARC layer 301 may be an organic ARC layer or an inorganic ARC layer.The inorganic ARC layer includes materials such as Ti, titanium oxide,titanium nitride, chromium oxide, carbon, amorphous silicon, siliconnitride, silicon nitride oxide, silicon carbon, and/or the like. Theorganic ARC layer includes photo-absorption materials and/or polymermaterials. In some cases, the organic ARC layer may include silicon. Inone embodiment, the ARC layer 301 is a bottom anti-reflection coating(BARC) layer.

The negative photoresist layer 302 is formed on the ARC layer 301 andthe positive photoresist layer 303 is formed on the negative photoresistlayer 302. The negative photoresist layer 302 has a thickness in a rangefrom about 500 Å to about 3000 Å. The positive photoresist layer 303 hasa thickness in a range from about 500 Å to about 3000 Å.

Each of the negative photoresist layer 302 and the positive photoresistlayer 303 is formed by, e.g., applying a spin-on process, in a sameprocess or a different process. Specifically, the spin-on process isperformed as follows. The negative photoresist layer 302 is coated onthe ARC layer 301 in a coating chamber; then, a first post applicationbaking (PAB) process is performed to bake the negative photoresist layer302 to remove some solvent therein; then, the positive photoresist layer303 is coated on the negative photoresist layer 302; and then, a secondPAB process is performed to bake the positive photoresist layer 303 toremove some solvent therein. In each PAB process, the temperature iscontrolled within a range from about 50° C. to about 200° C. for aprocessing time within a range from about 20 seconds to about 200seconds. After each PAB process, the substrate 300 can be cooled to theroom temperature.

In various embodiments, prior to coating the negative photoresist layer302 on the ARC layer 301, the substrate 300 and the ARC layer 301 aredehydrated in order to enhance the adhesion between the ARC layer 301and the negative photoresist layer 302. This dehydration may beperformed by treating the substrate 300 with hexamethyldisilazane gas ina high temperature environment.

The positive photoresist layer 303 includes, e.g., a resin, a photo acidgenerator (PAG), a base quencher, a solvent, an additive, and/or thelike.

The negative photoresist layer 302 may include a radiation-inducedcross-linking negative resist, a radiation-induced polymerizationnegative resist, or a radiation-induced polarity change negative resist.

In Step S203 of FIG. 8 and referring to FIG. 10, an exposure process isperformed to expose the negative photoresist layer 302 and the positivephotoresist layer 303.

As shown in FIG. 10, a photo mask 50 is provided, exposure light 32generated from an exposure system penetrates through portions of thephoto mask 50 to expose the negative photoresist layer 302 and thepositive photoresist layer 303. The exposure light 32 is configured tohave a first intensity threshold E1 and a second intensity threshold E2lower than E1. Under the influence of E1, a first exposure region 304 isformed in the positive photoresist layer 303. Under the influence of E2,a second exposure region 305 is formed in the negative photoresist layer302. A width “b” of the second exposure region 305 in the negativephotoresist layer 302 is greater than a width “a” of the first exposureregion 304 in the positive photoresist layer 303. As shown, the secondexposure region 305 in the negative photoresist layer 302 is under thefirst exposure region 304 in the positive photoresist layer 303.

Specifically, the photo mask 50 has a nonopaque part 51 and an opaquepart 52. The exposure light 32 generated from the exposure systempenetrates through the nonopaque part 51 to expose the positivephotoresist layer 303 and the negative photoresist layer 302.

In various embodiments, DUV light (i.e., deep ultra violet), such as Krfexcimer leaser (wave length: 248 nm), Arf excimer leaser (wave length:193 nm), or the like, can be utilized as the exposure light 32. F₂leaser (wave length: 157 nm), EUV (i.e., extreme ultra-violet) light(wave length: about 13.5 nm), or ultra violet glow generated from ultrapressure mercury lamps such as g-beam, i-beam, and the like, may also byutilized as the exposure light 32.

After the exposure light 32 penetrates through the nonopaque part 51 ofthe photo mask 50, the exposure energy in a space between the substrate300 and the photo mask 50 may include a sinusoidal distribution. Withina flat surface parallel to a surface of the substrate 300, the closer toa central axis of the nonopaque part 51, the higher exposure energy maybe as shown in FIG. 10. The exposure energy may reach a maximum valuenear the central axis of the nonopaque part 51. And due to the boundaryeffect of the opaque part 52, the exposure energy may degrade whendeparting from the central axis of the nonopaque part 51, and reach aminimum value near a central axis of the opaque part 52.

The distribution of the exposure energy after the exposure light 32penetrates through the photo mask 50 can be adjusted by tweaking powerof the light source and/or the critical dimensions of the opaque part 52and the nonopaque part 51.

On the energy distribution curve as shown in FIG. 10, a relatively highvalue is selected as the first intensity threshold E1, which may rangefrom about 70% to about 90% of a maximum value of the exposure energy;and a relatively low value is selected as the second intensity thresholdE2, which may range from about 10% to about 40% of the maximum value ofthe exposure energy. For example, a ratio of E1 to E2 ranges from about5:1 to about 2:1.

The exposure light 32 penetrates through the nonopaque part 51 to exposethe positive photoresist layer 303 to form the first exposure region 304in the positive photoresist layer 303. As described above, the positivephotoresist layer 303 includes a resin, a photo acid generator (PAG), abase quencher, a solvent, an additive, and/or the like. The PAG maygenerate a photo acid in response to an exposure energy greater than orequal to E1. If the energy is lower than E1, the PAG does not generatethe photo acid. The generated photo acid reacts with the resin in thepositive photoresist layer 303. Such reaction changes the first exposureregion 304, which is intrinsic insoluble, to be soluble. Therefore, thefirst exposure region 304 can be removed in a subsequently-performedpositive-tone development process. The base quencher in the positivephotoresist layer 303 is adapted for controlling the reaction, e.g., forterminating the reaction, between the photo acid and the resin in thepositive photoresist layer 303. The width “a” of the first exposureregion 304 equals to a width “c” of the distribution curve correspondingto the first intensity threshold E1 as shown in FIG. 10.

The exposure light 32 penetrates through the nonopaque part 51 to exposethe negative photoresist layer 302 to form the second exposure region305 in the negative photoresist layer 302. Chemical reactions may occurin the negative photoresist layer 302 in response to an exposure energygreater than or equal to E2. If the energy is lower than E2, thechemical reactions do not occur. For example, cross-linking reactionsmay occur in a radiation-induced cross-linking negative resist.Alternatively, polymerization reactions may occur in a radiation-inducedpolymerization negative resist. Therefore, the second exposure region305 in the negative photoresist layer 302, which is soluble before suchchemical reactions, is changed to be insoluble after such chemicalreactions. The insoluble second exposure region 305 cannot be removed ina subsequently-performed negative-tone development process. In contrast,other portions that are not exposed can be removed from the negativephotoresist layer 302. The width “b” of the second exposure region 305width “b” equals to a width “d” of the distribution curve correspondingto the second intensity threshold E2 as shown in FIG. 10.

In this manner, the chemical reaction in the negative photoresist layer302 to form the second exposure region 305 therein may occur in responseto the exposure energy greater than or equal to E2. The photo acid(along with its chemical reaction) in the positive photoresist layer 303may be generated in response to the exposure energy greater than orequal to E1. E2 is lower than E1. On the distribution curve shown inFIG. 10, the width “d” corresponding to E2 is greater than the width “c”corresponding to E1. Therefore, after the exposure, the width “b” of thesecond exposure region 305 is greater than the width “a” of the firstexposure region 304.

For example, the second exposure region 305 may have the width “b”approximately 1.5 times to 4.5 times (e.g., about 3 times) of the width“a” of the first exposure region 304. The widths of the exposure regionscan be easily adjusted as desired. For example, when E1 and E2 arepredetermined, the distribution of the exposure energy can be adjustedby adjusting power of the light source and/or the critical dimensions ofthe opaque part 52 and the nonopaque part 51. The widths on thedistribution curve respectively corresponding to E1 and E2 can beadjusted. Thus, the second exposure region 305 and the first exposureregion 304 may have desired widths after exposure.

In an exemplary embodiment, the second exposure region 305 may have thewidth “b” of about 3 times of the width “a” of the first exposure region304, so that a double patterned negative photoresist layer includingcomponents with the same linear widths and pitches may be obtained.

In another exemplary embodiment, the second exposure region 305 may havethe width “b” of less than about 3 times of the width “a” of the firstexposure region 304, so that the linear widths of the components may besmaller and the pitches may be larger. To the contrary, in anotherexemplary embodiment, the second exposure region 305 may have the width“b” of more than about 3 times of the width “a” of the first exposureregion 304, so that the linear widths of the components may be largerand the pitches may be smaller.

After the exposure process, a post exposure baking (PEB) process may beperformed to the positive photoresist layer 303 and the negativephotoresist layer 302 to further control widths of the exposure regionsand eliminate the standing wave effect.

In the PEB process, the temperature is controlled within a range fromabout 50° C. to about 200° C., for a processing time within a range fromabout 15 seconds to about 200 seconds. After the PEB process, thesubstrate 300 can be cooled to the room temperature.

In Step S204 of FIG. 8 and referring to FIG. 11, a positive-tonedevelopment process is performed to remove the first exposure region 304from the positive photoresist layer 303 to form one or more firstopenings 306 in the resulting positive photoresist layer 303 a. Thefirst openings 306 expose the negative photoresist layer 302.

The positive-tone development process uses an aqueous alkali solution asthe developer, which is also referred to as an aqueous alkali developer.The first exposure region 304 is soluble in the aqueous alkali developerand the other portions of the positive photoresist layer 303 areinsoluble. Therefore, the first exposure region 304 is removed in thepositive-tone development process to form the first openings 306 asshown in FIG. 11.

Alkali materials in the aqueous alkali developer may include one or moreselected from: sodium hydroxide; potassium hydroxide; sodium carbonate;sodium silicate; sodium metasilicate; aqueous ammonia; primary aminessuch as ethylamine or n-propylamine; secondary amines such asdiethylamine; tertiary amine such as triethylamine; alcoholamine such asdimethylethano; quaternary ammonium salt such as tetramethylammoniumhydroxide (TMAH) and tetraethylammonium hydroxide (TEAH); and/or cyclicamine. In an embodiment, the aqueous alkali developer is TMAH. Invarious embodiments, the aqueous alkali developer may have a pH valuewithin a range from about 9 to about 15.

The processing time period of the positive-tone development processusing the aqueous alkali developer may be within a range from about 10seconds to about 300 seconds. After the development, the substrate 300may be rinsed with purified water. Note that the positive-tonedevelopment process does not affect the negative photoresist layer 302underlying the positive photoresist layer 303.

After the positive-tone development process, a thermal treatment may beperformed to treat the substrate 300 to remove remaining water andsolution to further enhance the adhesion between the positivephotoresist layer and underlying material.

In Step S205 of FIG. 8 and referring to FIGS. 12-13, the second exposureregion 305 of the negative photoresist layer 302 is etched along thefirst openings 306 using the remaining positive photoresist layer 303 aas a mask to form one or more second openings 307 to expose the ARClayer 301. Thereafter, the remaining positive photoresist layer 303 a isremoved.

Etching the second exposure region 305 in the negative photoresist layer302 may be performed by a reactive ion etching process, e.g., usingoxygen as an etching gas. In an embodiment, the etching gas furtherincludes an inert gas including, such as, for example, He, Ne, N₂, Ar,Xe, or a combination thereof.

In an embodiment, removing the remaining positive photoresist layer 303a and etching the second exposure region 305 in the negative photoresistlayer 302 may be performed simultaneously in a same etching process.Therefore, processing steps and cost are further saved. In some cases,to ensure that the ARC layer 301 is exposed through the second openings307, the negative photoresist layer 302 may be over etched during theetching process.

In other exemplary embodiments, the positive photoresist layer 303 a maybe removed in a separate process from etching the negative photoresistlayer 302 to expose the ARC layer 301.

For example, the positive photoresist layer 303 a may be removed in asubsequently performed negative-tone development process if thedeveloper used in the negative-tone can dissolve the positivephotoresist layer 303 a. Processing cost and steps can be reduced.

In Step S206 of FIG. 8 and referring to FIG. 14, a negative-tonedevelopment process is performed to remove portions of the negativephotoresist layer 302 outside of the second exposure region 305 to forma double patterned negative photoresist layer 302 a.

In the negative-tone development process, the second exposure region 305is insoluble, while the other portions outside of second exposure region305 in the negative photoresist layer 302 can be dissolved into thedeveloper. Therefore, after the negative-tone development process, theremaining second exposure region 305 may form a double patternednegative photoresist layer 302 a as shown in FIG. 14.

In various embodiments, the negative-tone development process utilizesorganic solution as the developer which is also referred to as anorganic developer. The organic developer includes one or more selectedfrom: ketone solvent such as octanon and the like; ester solvent such asbutyl acetate, amyl acetate, Ethyl 3-Ethoxypropionate, butyl formate,propyl formate, and the like; alcohol solvent such as n-propyl alcohol,isopropyl alcohol, butyl alcohol, hexyl alcohol, heptyl alcohol, octylalcohol, and the like; and/or glycol ether solvent such as ethyleneglycol monomethyl ether, ether solvent, and the like. Optionally, theorganic developer may further include surfactant. After thenegative-tone development process, the substrate 300 may be rinsed withany suitable organic solutions.

In various embodiments, the negative-tone development process may alsouse the aqueous alkali developer as used in the positive-tonedevelopment process. In one embodiment, the aqueous alkali developer isTMAH. The aqueous alkali developer may have a pH value within a rangefrom about 9 to about 15. The processing time of the negative-tonedevelopment process using the aqueous alkali developer may be within arange from about 10 seconds to about 300 seconds. After the development,the substrate 300 is rinsed with purified water.

After the negative-tone development process, a thermal treatment may beperformed to the substrate 300 to remove remaining water and solution tofurther enhance the adhesion between the negative photoresist layer andunderlying material.

In an exemplary embodiment, when the negative photoresist layer 302includes a radiation-induced polymerization negative resist or aradiation-induced polarity change negative resist, the negative-tonedevelopment process may use the organic developer as disclosed herein.In another exemplary embodiment, when the negative photoresist layer 302includes a radiation-induced cross-linking negative resist, thenegative-tone development process may use the organic developer or theaqueous alkali developer.

In Step S207 of FIG. 8 and referring to FIGS. 15-16, the ARC layer 301and the target layer 310 are etched using the double patterned negativephotoresist layer 302 a as a mask to form a double patternedsemiconductor structure having openings 308 as shown in FIG. 15.Remaining ARC layer 301 may then be removed to leave a patterned targetlayer 310 a having a plurality of target layer structures 309 as shownin FIG. 16. Any suitable etching processes may be used to etch andremove the ARC layer 301 and/or the target layer 310.

In this manner, the disclosed method uses one exposure process and onephoto mask to save the processing cost. In the one exposure process, afirst exposure region and a second exposure region are respectivelyformed in the positive photoresist layer and the negative photoresistlayer. A pattern of the positive photoresist layer is transferred to thenegative photoresist layer by an etching process, and the pattern of thenegative photoresist layer is formed by a negative-tone developmentprocess. Therefore, a double patterned structure is formed in thenegative photoresist layer. The process is simple with high accuracy. Inaddition, because development processes are respectively performed tothe negative photoresist layer and the positive photoresist layer, thereare no interactions between the development processes. Such processesare easily controlled with good topography for obtaining the doublepatterned structure.

In one embodiment, removing the remaining positive photoresist layer andetching the second exposure region in the negative photoresist layer (asshown in FIGS. 12-13) can be performed simultaneously in the sameprocess. Alternatively, the remaining positive photoresist layer may beremoved in a negative-tone development process. Processing steps andmanufacturing cost may further be reduced.

The embodiments disclosed herein are exemplary only. Other applications,advantages, alternations, modifications, or equivalents to the disclosedembodiments are obvious to those skilled in the art and are intended tobe included within the scope of the present disclosure.

What is claimed is:
 1. A method for forming a semiconductor structure,comprising: providing a substrate; forming a negative photoresist layerover the substrate; forming a positive photoresist layer on the negativephotoresist layer; performing an exposure process to form a firstexposure region in the positive photoresist layer and a second exposureregion in the negative photoresist layer, wherein the first exposureregion having a first width is located directly on the second exposureregion having a second width, and the first exposure region and thesecond exposure region have a same central axis according to theexposure process, and wherein the first width is less than the secondwidth; performing a positive-tone development process to remove thefirst exposure region from the positive photoresist layer to form one ormore first openings to expose the negative photoresist layer; etchingthe second exposure region of the negative photoresist layer along theone or more first openings to form one or more second openings throughboth the positive photoresist layer and the negative photoresist layer;removing remaining positive photoresist layer; and performing anegative-tone development process to remove portions of the negativephotoresist layer outside of remaining second exposure region to form adouble patterned negative photoresist layer.
 2. The method according toclaim 1, wherein the exposure process provides an exposure energy havinga first intensity threshold and a second intensity threshold, the firstintensity threshold being greater than the second intensity threshold.3. The method according to claim 2, wherein: the first exposure regionis formed in response to the exposure energy greater than or equal tothe first intensity threshold, the second exposure region is formed inresponse to the exposure energy greater than or equal to the secondintensity threshold.
 4. The method according to claim 1, wherein thepositive photoresist layer comprises a photo acid generator and a resin,the photo acid generator generating a photo acid when the exposureenergy is greater than or equal to the first intensity threshold, thephoto acid reacting with the resin to form the first exposure region. 5.The method according to claim 1, wherein the negative photoresist layerincludes a radiation-induced cross-linking negative resist, aradiation-induced polymerization negative resist, or a radiation-inducedpolarity change negative resist.
 6. The method according to claim 1,wherein the second width of the second exposure region is about 1.5times to about 4.5 times of the first width of the first exposureregion.
 7. The method according to claim 1, wherein the positive-tonedevelopment process uses a developer comprising an aqueous alkalisolution, and the first exposure region of the positive photoresistlayer is soluble in the aqueous alkali solution and the remainingpositive photoresist layer outside of the first exposure region areinsoluble in the developer.
 8. The method according to claim 1, whereinthe negative-tone development process uses a developer comprising anorganic solution or an aqueous alkali solution, and the second exposureregion of the negative photoresist layer is insoluble in the developerand the portions of the negative photoresist layer outside of theremaining second exposure region are soluble in the developer.
 9. Themethod according to claim 1, wherein the etching of the second exposureregion of the negative photoresist layer along the one or more firstopenings comprises a reactive ion etching process, the reactive ionetching process using oxygen as an etching gas.
 10. The method accordingto claim 1, wherein the removing of the remaining positive photoresistlayer and etching the second exposure region of the negative photoresistlayer along the one or more first openings are performed in a sameprocess.
 11. The method according to claim 1, wherein the remainingpositive photoresist layer is removed together with the second exposureregion in the negative-tone development process.
 12. The methodaccording to claim 1, further comprising: performing a post exposurebaking process to the substrate after the exposure process.
 13. Themethod according to claim 1, wherein each of the positive photoresistlayer and the negative photoresist layer is formed in a spin-on processcomprising: forming the negative photoresist layer over the substrate;performing a post application baking process to the negative photoresistlayer; forming the positive photoresist layer on the negativephotoresist layer; and performing a second post application bakingprocess to the positive photoresist layer.
 14. The method according toclaim 1, further including simultaneously performing the etching of thesecond exposure region of the negative photoresist layer and theremoving of the remaining positive photoresist layer.